Positive matrix factorization of ultrafine particle mass (PM_0.1) at three sites in California

Ultrafine particles (UFPs) are an emerging air quality concern because of their enhanced toxicity compared to larger airborne particles. This study aims to better understand source contributions to UFP mass (PM_0.1) at multiples sites across California. Three-day average samples of PM_0.1 collected over a full year at San Pablo, East Oakland, and Los Angeles were analyzed using Positive Matrix Factorization (PMF). Seven PM_0.1 source-factors were identified at all locations: Factor1- Gasoline+Motor Oil+Meat Cooking+Natural Gas+SOA (31-53% PM_0.1 mass), Factor 2- Diesel+Motor Oil (25-45% PM_0.1 mass), Factor 3-Wood Burning (6-12% PM_0.1 mass), Factor 4-Shipping and other heavy fuel oil combustion (2-3% PM_0.1 mass), Factor 5-Sea Spray (4-8% PM_0.1 mass), Factor 6-Sb Brake Wear (1-3% PM_0.1 mass) and Factor 7-Sn - Unknown (1-7% PM_0.1 mass). PM_0.1 wood burning contributions were highest in the winter season when residential wood combustion was active. The monthly-averaged PM_0.1 source apportionment results calculated by PMF are consistent with the PM_0.1 source apportionment results calculated using Chemical Mass Balance (CMB) from the same sampling campaign. PMF distinguished Diesel+Motor Oil from Gasoline+Motor Oil+Meat Cooking+Natural Gas+SOA based on the species EC3 (a sub-fraction of elemental carbon that is volatilized and oxidized at temperatures between 700 and 775 °C), but PMF failed to further resolve the major sources of PM_0.1 OC because unique tracers were not measured. PMF resolved "Shipping and other heavy fuel oil combustion" and Sea Spray sources based on inorganic tracers V and Br. The PMF factor rich in Sb very likely comes from brake wear associated with on-road vehicles and railway operations. The undefined Sn factor may be indicative of local industrial sources and traffic emission, but further research will be required to confirm this hypothesis. The PM_0.1 source apportionment results contained in the current study further characterize the seasonal and spatial patterns of UFP concentrations in California.